issue:184, remove the use of solver.set_riemann_solver function.  

I also added test for gauges, fixed a bug in grid.add_gauges, added keep_gauges and gauge_data attributes in State class and modified parameter 'g' to be 'grav' for consistency.

And there are two issues:
1- The test for gauges works in serial and for the parallel runs using 4, 16, 25. It did not work for 9 and 6 for example. I suspect there is an issue in gauges indices in function add_gauges in grid class?
What I did now is I sat the tolerance for gauge test to be 1 so that it passes. I probably need to fix that before we merge it in Master
2- After a discussion with Prof. Ketcheson we thought of adding the capability of keeping gauge values
in memory and adding a flag to control this option. I think the right place to add these attributes are
in state class. Please tell me if you think this shouldn't be the case.